Numerous methods and apparatus exist in the art for measuring chemical components of fluids, and current technology utilizes many types of sensors for detecting components, analytes, in numerous types of fluids. For example, some of these range from oxygen sensors for detecting oxygen in air for control of the air and fuel ratio for combustion in internal combustion engines to multiple phase sequential analyzers for qualitative and/or quantitative measurement of constituents or analytes of blood. For instance, the measurement of blood gases, usually a measure of the partial pressures of oxygen and carbon dioxide, along with the pH from a sample of arterial blood gives the state of the acid base balance or the effectiveness of both the respiratory and cardiovascular systems of the human or vertebrate body. These various types of sensors can be prepared by various techniques including layered circuit or integrated circuit technologies, as for example, thick film, thin film, plating, pressurized laminating and photolithographic etching, and other like silk screening processes.
In many of the existing measurement methods, when the fluid is a liquid or liquid with a dissolved gas with or without the presence of solids, it may be necessary to transport a sample to a central location for testing. With centralized testing, the bulky, stationary, elaborate and sophisticated equipment performs the analysis on a practically endless number of samples. An example of this is the qualitative and/or quantitative measurement of constituents or analytes of blood. For instance, the measurement of blood gases, usually a measure of the partial pressures of oxygen and carbon dioxide, along with the pH from a sample of arterial blood gives the state of the acid base balance or the effectiveness of both the respiratory and cardiovascular systems of the human or vertebrate body. For measuring constituents of blood, the blood sample is drawn from the patient and usually, as in the case of blood gases, transported to a central location for testing.
This technique of transporting the sample to stationary measuring equipment can lead to problems. Ingenious technology has broached solutions to maintain the original composition of the fluid during transportation. Elaborate designs for syringes used in taking the blood samples overcame some problems that resulted in inaccurate readings of the particular chemical constituent being measured. For instance, in determining blood gas composition, the problem of air contamination in the collected sample was solved by the use of liquid heparin as an anticoagulant. Unfortunately, this introduced a sample dilution problem. Subsequent development resulted in the use of heparin in the dry state as opposed to the liquid state to avoid this dilution. Also, elaborate designs provided for proper mixing of the sample after transportation but before testing. Even with these improvements, there are many reports in the literature that suggest that the values obtained in the measurement of blood gases depend on the type of measuring equipment and the technique for sample collection.
The art also has attempted to develop more portable measuring equipment rather than the fairly expensive nonportable equipment that engender the elaborate and cumbersome transportation techniques. Devices that are very portable could shorten or overcome transporting the sample altogether so that a patient's blood gases could be measured at the bedside in a manner similar to measuring a patient's temperature. U.S. Pat. Nos. 3,000,805 and 3,497,442 show two such devices. The former has electrodes located on a syringe plunger and the latter has electrodes placed on the syringe well to conduct the measurements. The electrodes are the sensing devices for the blood gases. In the allowed U.S. patent application Ser. No. 07/343,234, now U.S. Pat. No. 5,046,496. Applicants assignee describes and claims a portable blood gas sensor which includes electrodes fabricated from a conventional silk screening process where the electrodes are screened on to a ceramic substance. Typically these electrodes are used along with an electrolyte and analyte permeable membrane that covers the sensor. Some of these membranes may be hydratable membranes that can be stored in a dry state and hydrated just prior to use.
The utilization of portable equipment to obtain accurate analysis reports while using a disposable device could be advanced with improvements in electronic circuit board design.
Accurate sensing of the ambient temperature of the wiring board is required to precisely control the heater to ultimately maintain, within a narrow distribution of temperatures, the desired operating surface temperature on the wiring board in the region the several sensors. Also the accurate sensing of temperature is important is the area of measuring two phase calibrant liquids so that the calibrant values can be corrected for the most recent storage temperature.
Placement of the all of the components, including the heater, on the wiring board is also very important to obtain the maximum utility and capability of these components and minimize power consumption.